The present invention relares to a magnetic recording and reproducing system for recording video signals on a magnetic tape and reproducing the recorded signals and more particularly to a magnetic recording system for intermittently recording successive television camera signals or television signals.
In a magnetic recording system for recording the video signals produced by a television camera or the video signals produced by receiving a television program, a magnetic tape is helically wrapped more than 180 degrees around a drum for guiding the magnetic tape and the signals are recorded on the magnetic tape by a plurality of rotary video heads. In this type of magnetic recording system, the video signal for one field is successively recorded on each of video tracks formed on a magnetic tape in the form of recording paths which are each inclined at a given angle with respect to the lengthwise direction of the tape.
With this type of magnetic recording system, attempts have recently been made in which the video signals produced by a television camera or the like are intermittently recorded at intervals of a predetermined time on a magnetic tape to make a long-time recording on the single reel of the magnetic tape. This system has been used as a camera monitoring system for recording, for example, the video signals from a burglar-monitor camera or for the purpose of recording information such as documents.
In the case that such an intermittent recording is desired, differing from the ordinary continuous recording, the magnetic tape is run intermittently so that during the recording the movement of the magnetic tape is stopped and in this rest condition the video signal for one field is recorded on one video track and then the recording is stopped until the next recording time is reached. During this rest or pause period the magnetic tape is moved by one video track and the video signal for deriving one field is recorded at the next recording time. By thus effecting the field skipping, it is possible to record long-play video signals on a single reel of the magnetic tape. To reproduce thus recorded magnetic tape, the plurality of video heads repeatedly reproduce a number of times each of the video tracks and reproduce it as a still picture.
Then, since the video signals obtained from a television camera or a television program are video signals produced by interlaced scanning, the period of the video signal for one field becomes 262.5 H (H represents one horizontal period) according to the NTSC system. (In the discussion to follow, the parenthesized figures relate to the case of 312.5 H according to CCIR system). As a result, where the same video track is repeatedly reproduced by two video heads alternately and the output signals of the video heads are joined to produce a reproduced video signal, the horizontal synchronizing signal spacing becomes 0.5 H (0.5 H) at the joint of the output signals from the video heads and this causes a skew to appear in the reproduced picture.
In the past, in order to prevent the occurence of such skew, an attempt has been made in which during the reproduction the output signal from one of two reproducing magnetic heads are passed through a delay circuit having a 0.4 H delay time and the resulting video signal delayed by 0.5 H and the output signal from the other magnetic head are alternately reproduced by switching thereby ensuring a time interval of 1 H at the time of switching of the heads.
However, this type of reproducing system employs a glass delay element as a delay circuit so that if the video signal is simply delayed by the glass delay element, the quality of the video signal is deteriorated due to the characteristics of the glass delay element. Thus, in order to prevent any deterioration of the quality of the video signal, it is necessary to supply the video signal to a balanced modulator which converts the video signal to a high frequency band and then supply the signal to the glass delay element thus requiring a video signal processing circuit. Also, the use of the video signal processing circuit tends to cause the signal level and frequency characteristic of its output video signal to differ from those of the output video signal from the other magnetic head and this in turn results in a flicker phenomenon in the reproduced picture. Also, due to the fact that in the signal processing circuit the balanced modulator includes an oscillator circuit, there is a disadvantage that a sufficient electrostatic shielding must be provided for the oscillator circuit with the resulting increase in the size of the circuits and mechanisms of the reproducing system.